Any time a cable is connected to a computer system (e.g., through USB, FireWire, or other common input/output ports) there is a risk of damage to the system resulting from a Cable Discharge Event (CDE.) A CDE results from static charge having accumulated on the cable and being discharged to the computer system when the cable is connected to the computer system. For example, in many office settings, personnel may be moved from one location to another to re-task the personnel, move locations, or the like. Computers for the personnel may be moved along with the personnel and reconnected to a network at the new location. Moving cable with a isolated pins and shielding can often build up an electrostatic charge as the cables rub against one another, rub against the carpet or wall, or even as materials within the cable rub against one another.
Electrostatic charges that build up on the cables can vary significantly in voltage depending upon the relative humidity and the materials involved. For instance, just walking across a carpeted area when the relative humidity is about 65% to 90% can typically generate an electrostatic charge of 1,500 volts. Walking across the same carpeted area when the relative humidity is approximately 10% to 20% humidity can generate an electrostatic charge of 35,000 volts.
ESD is a serious issue in electronic systems. When a statically-charged cable is connected to an electrostatic discharge sensitive (ESDS) electronic system, there is a possibility that the electrostatic charge may discharge through sensitive circuitry in the electronic system. High voltages can damage or degrade insulating materials and, if the electrostatic discharge possesses sufficient energy, damage could occur due to localized overheating. In general, devices with finer geometries are more susceptible to damage from ESD.
Integrated circuits (ICs) are particularly susceptible to ESD, especially when considering the drive to build ICs with smaller geometries in successive generations. ICs are made from semiconductor materials such as silicon and insulating materials such as silicon dioxide, which can break down if exposed to high voltages. Manufacturers and users of ICs must take precautions to avoid this problem. Such measures include appropriate packing material, the use of conducting wrist straps and foot-straps to prevent high voltages from accumulating on workers' bodies, anti-static mats to conduct harmful electric charges away from the work area, and humidity control.
Designers of computer systems typically attempt to protect their products from CDE damage by incorporating electrostatic discharge (ESD) protection structures into the components used in their systems; in the event of a CDE, these ESD protection structures are designed to route the charge from the cable to ground and thus avoid or attenuate damage to the protected components.
In practice, however, the use of ESD protection devices on components offers only limited protection. Individual ESD structures vary in their ability to handle ESD events, and can wear out over time from handling ESD events. Severe CDEs can easily exceed the capabilities of even the best ESD protection structures and cause immediate and catastrophic damage to computer systems. For example, many ESD protection devices can handle up to approximately 2,000 volts but are damaged in the event of a higher voltage ESD.
Once a computer system has been manufactured and sold, there is no feasible option for changing its internal design or structure to improve its resistance to CDEs.